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In a brand baby spanking new world I am building, appropriately named Escargard, punningly named by the AndyD273, mollusks have become the dominant form of life. Resulting in aliens similar to those below

enter image description here

But sadly the longer I thought of such a scenario, the less likely It became, while many mollusks lack the scaling problem that insects face, they have their own. The gelatinous structure of the body of mollusks makes it difficult to rightly scale them.

Without editing their biology, what is the largest I can make a mollusk? What changes can I make to mollusk biology to allow them to become massive?

kingledion
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TrEs-2b
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  • Just to clarify, in these sketches there seem to be humans for comparison, suggesting you are looking for land based creatures sized like dinosaurs und earth-like conditions of gravity and atmosphere? – Durandal Dec 07 '16 at 21:33
  • @Durandal The humans are for size and I am hoping for the planet to be relatively earth like – TrEs-2b Dec 07 '16 at 21:36
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    Water-borne mollusks in Earth's fossil history approach those sizes, but not land creatures. – John Feltz Dec 07 '16 at 21:41
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    @JohnFeltz You don't even have to go to prehistoric mollusks. The Giant Squid is one of the largest animals alive today. – Mike Nichols Dec 07 '16 at 22:20
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    Philosophical questions like "do we have souls" and "what defines consciousness" pop up on here all the time but "what is a mollusc" is something new – Zxyrra Dec 08 '16 at 00:50
  • Or the Giant Clam: https://en.wikipedia.org/wiki/Giant_clam The scaling only becomes a major problem for land-dwelling creatures. E.g. crabs are exoskeletal, like insects, and grow much larger than land dwellers: https://en.wikipedia.org/wiki/Japanese_spider_crab – jamesqf Dec 08 '16 at 06:02
  • Spider Crab.....on the one hand I wonder if it is tasty, on the other hand my arachnophobia has me wanting to jump on my chair. – NZKshatriya Dec 08 '16 at 06:55
  • Related: Can you simply scale up animals? – user Dec 08 '16 at 12:42
  • https://www.youtube.com/watch?v=5N920UYc3Y0 – David K Dec 08 '16 at 14:46

7 Answers7

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Yes, depending on where you draw the line on 'mollusc.'

A more accurate term might be 'mollusc-descended.' Any very large land mollusc would almost certainly no longer be invertebrates, since after a certain size, the square-cube law would dictate that their unsupported insides collapse into jelly. Even in the picture you provide, some of the 'molluscs' have legs, which means that they have bones. They would also most likely develop some muscle-analogue to make better use of its new bones. Larger mollusc descendents might resemble animals with integrated shells such as turtles or ankylosaurs, and have a mixture of traits from both of them.

Their characteristic breathing cavity in their mantle is pretty primitive, so there would likely have to be substantial changes to that in order to make it able to handle larger sizes, unless there was enough oxygen in the atmosphere to make fire a huge hazard (which could provide an interesting reason for most life to remain in the sea, and provide unique challenges for life that elects to stay on land).

Sea-based molluscs would have no such restriction due to the support of the water around them, and would be free to assume whatever twisted, nightmarish forms your imagination can devise.

Without making any changes to their biology, I doubt you'd be able to make land-based molluscs bigger than human-scale, since that's about where prehistoric insects peaked at. And even that seems a little far-fetched. Squishy invertebrates lacking exoskeletons simply do not scale well without major changes.

Community
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emo bob
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    This is good. Convergent evolution is a thing. Looking at examples of mammals or dinosaurs that have some features in common with molluscs and then plotting a path of convergent evolution is a good approach. – kingledion Dec 07 '16 at 23:30
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    "Squishy invertebrates lacking exoskeletons simply do not scale well without major changes." - they wouldn't need to scale anything, they could flow across the landscape instead :-) – John Dvorak Dec 08 '16 at 15:31
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This is a mollusk.

enter image description here

It's a 7 meter long giant squid preserved in a block of ice.

There's an even larger species of mollusk, the Colossal squid, living in the Antarctic Ocean. It's estimated to be about 13 meters long and weighing in at about 750kg. Its eyeball is a foot wide.

Another example of a large mollusk is the Seven-Arm Octopus clocking in at 3.5 meters long and weighing about 75 kg, as much as an adult human.

If you're wondering where the shell is, it's on the inside. It seems the trick for a mollusk to get big is to ditch the shell and use it as a supporting structure. Large animals can use their sheer bulk as protection.

Schwern
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Land dwelling molluscs are possible, but will change their structure considerably in order to function as fully land dwelling creatures. Even amphibious molluscs will need to compromise the features which make them successful aquatic creatures in order to be functional on land.

Some things to consider if they are the truly dominant order on the planet is they will have radiated out to fill lots of different niches (as suggested by the diagrams). However, just drawing a creature because it is "cool" isn't really going to explain why it looks the way it does. Consider how terrestrial animals have evolved to fill the various niches, then realize your molluscs will have to create some similar adaptations to fulfill similar roles in the ecosystem.

Here are two illustrations from the TV show "The Future is Wild", which postulates squids coming to live on land 200 MY in the future. The "Megasquid" is a huge creature which seems to fill the role of large herbivore, much like an elephant or maybe sauropod dinosaur, so it has grown to an incredible size, some of its limbs have bulked up to support terrestrial locomotion, its gut may have subdivided into multiple stomachs to digest plant matter (or it has a gizzard full of rocks) and so on.

enter image description here

enter image description here

Megasquid

Another postulated creature is the Squibbon, a tree dwelling creature perhaps analogous to monkeys. It may not look like a monkey, but it has adapted in many similar ways to living in trees, including long grasping limbs, moving the eyes for stereoscopic vision and so on:

enter image description here

Squibbon

You could do similar thought processes for major predators, small creatures which fulfill the roles of mice, molluscs which have moved back to the sea in the manner of seals or whales and so on.

So have fun with this, but think carefully about the ecological role your creature is supposed to fulfill, look at how earthly life has done so (over several eras, there have been giant amphibians, dinosaurs, marsupials and mammals, as well as occasional birds which have all filled similar roles in the ecosystem, so you have lots of examples to look at).

Hypnosifl
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Thucydides
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    A technical point on the so-called Megasquid, also relevant to other creatures you postulate..... It is described as having muscles which "bind" as columns, to take the function of bones, and a novel (for earth) gait. If the 8 legs are labelled L1-L4 (left) and R1-R4 (right) the suggested gait is L1+R2+R3+L4 all move, then R1+L2+L3+R4 all move. – Stilez Dec 08 '16 at 03:57
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    This is the second time I've seen a squibbon posted at Worldbuilding. Whats going on here, some sort of invasion? – kingledion Dec 08 '16 at 04:18
  • @kingledion: Look to the trees...... – Thucydides Dec 08 '16 at 14:55
  • I love the squibbon... – John Dvorak Dec 08 '16 at 15:33
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    Apparently the megasquid was designed by a professor of zoology who did some calculations to show that its size was plausible, see https://www.theguardian.com/uk/2003/jun/28/highereducation.science – Hypnosifl Aug 01 '19 at 03:22
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All you need is a support structure, your mollusks need to evolve a skeleton of some kind for life on land. they already have the tissue to lay down calcium carbonate so you could have a calcium carbonate based skeleton.

Mollusks have a closed circulatory system, so unlike insects, their size is really just limited by how they can support themselves. If you really want to keep the shell you could make them essential somehow, maybe in the aliens they provide the rigid space and attachment for the muscles that pull on lungs, and thus it is easier to keep them then evolve a new breathing system.

for the aquatic species you are fine, some ammonites got huge while keeping their shell. some believe the shells may have even become buoyancy chambers. And some of the heteromorph ammonite have truly bizarre shapes. honetsly google image search "heteromorph ammonites" there are just so many shapes. enter image description here

John
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Maybe, but they won't look like the ones you've drawn

What is a mollusk? It's safe to assume that these creatures aren't actually Mollusks, since that would require that they be descendant from the same common ancestor as Earth-mollusks, so let's go with mollusk like aliens. Let's say that they're all descendant from some mollusk-like ancestor, but that without competition from some tetrapod-like creature, they've gone on to evolve and radiate and occupy most niches that tetrapods have gone on to inhabit on Earth.

That's certainly possible, but your final forms probably won't look much like mollusks. Over time, they'll gain adaptations to help them survive in a variety of different land environments, increase in size, move about, see, and do everything else that's useful for large land animals.

Eye stalks, for example, are likely to vanish as the animals become more mobile and develop more complex eyes. They're great if you're a snail, since snails can't turn around to see what's behind them, but for a legged, running animal, they offer little advantage, are metabolically costly to grow, and put fairly delicate, difficult to regenerate structures (the eyes) in danger of damage. Shells, similarly, will be lost in any animal that relies on running down its prey, running to escape predators, or that's so big it doesn't need to worry about predators. Mobility, in an environment of relatively immobile creatures, is a huge evolutionary advantage. Early fish, in a similar example, had thick plates of bone armoring their bodies, but these were lost in favor of small scales for the sake of increased mobility. Other adaptations will appear as well. Things like hard shelled eggs, live birth, dry skin, and internal bone-like structures will all appear, allow their bearers to live in dry climates and support their bulk out of water.

That being said, while certain niches strongly favor creatures with these types of adaptations, it will almost definitely be the case that there exist other evolutionary niches in which less advanced (and more mollusk-y) creatures can compete easily, or even gain an advantage. Jawed, sluglike creatures with primitive skeletons and tentacles might make for effective amphibious ambush predators, similar to crocodiles on Earth, while snail-like creatures with supportive internal structures and advanced lungs might be perfectly suited to shuffle around in shallow waterways and marshes, mowing through patches of aquatic plants. Just like amphibians on Earth, though, they'll likely be replaced as the dominant creatures on your planet as evolution produces more highly evolved forms that can out compete them in other niches.

ckersch
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  • After a time, the descendants would have little-to-no resemblance to molluscs. I wonder what evolutionary pressures could make it advantageous for most creatures to retain their mollusc traits. Perhaps periodic bad conditions with few places to hide, thus incentivizing having a shell to hunker down in? That could create a dichotomy between the small mammal-like creatures that aren't very mollucsy but are good at hiding, and the big, slow mollusc creatues that are good at enduring. – emo bob Dec 07 '16 at 23:39
  • Either way, ckersch makes an excellent point about how it's unrealistic to expect all creatures to maintain mollusc traits. – emo bob Dec 07 '16 at 23:41
  • To avoid it coming back to bite me in the ass, I didn't draw these, this artist did: http://spighy.deviantart.com/art/Alien-snails-630838856 – TrEs-2b Dec 08 '16 at 00:46
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Giant molluscs - particularly cephalopods - are frequent subjects in speculative evolution. Another answer talks about the Megasquid and Squibbon from TFiW, but there are many more I can think of from hobbyist SE projects. For example:

The Stalker, a predatory descendant of the cuttlefish:

enter image description here

The Desert Hopper, a macropod-like snail:

enter image description here

Dragonsquids, cephalopods that fly with their lateral flaps:

enter image description here

Believe me, there's a lot more where that came from.

So, giant molluscs are very much plausible, at least according to many speculative evolution fans. But how? Aspiring speccer vcubestudios wrote this on Deviantart:

"Rising out from swamps and marshes, a kind of convergent evolution has taken place, calling back to the days of tiktaalik and protostega. But these were not lobe finned fish, nor were they amphibians. They were octopuses, cuttlefish and squid, mimicking the course of evolution taken by vertebrates eons ago. They waded about in the shallows, slowly creeping onto land. Their shells developed inside of their bodies, even becoming like limbs to support their weight on the earth."

Alas, I'm not going to simply cram other people's information into your face and leave. I too am a practitioner of much speculative evolution, so I'll lend some of my own theories to you.

The accepted answer tells you that you couldn't get giant molluscs past human-size. That is correct - if the terrestrial molluscs had no skeleton. But that answerer evidently didn't consider the possibility of internal shells.

The snails, the nautiloids, and the ammonoids and belemnoids of old are all shelled molluscs. It is by no means unreasonable to think that other mollusc groups could A) evolve shells and B) internalize them. This would serve as an analog of an endoskeleton.

I recommend you look at Wikipedia's page on human evolution, to show you how life crawled out of the land and came to rule it. But, a mollusc domination of the planet won't be so similar to ours.

What could be really interesting about your future world is that there could be animals with anywhere from one foot (Gastropods) to ten feet (Cephalopods). If you want cephalopods to colonize the land, they'd need to evolve tougher skin and a more adaptable respiratory system as well as that endoskeleton, but they're a very advanced group so I wouldn't put it past them.

In the water, molluscs could also evolve giant forms, much more so than on land. One thinks of Spec Dinosauria's Balaenoteuths, squids that evolved into the niche of whales in the absence of mammals:

enter image description here

That's about all I've got for now. I know that you've accepted an answer, but I thought it didn't really consider the possibility of pseudo-endoskeletons, which could dramatically increase the size of your "sandpit" when considering giant molluscan forms.

I also highly recommend you look at some of these pages (Particularly the one on Balaenoteuths, and that youtube presentation), as they will doubtlessly give you countless ideas.

I hope this helps, and wish you every fortune in your new worldbuilding project. If you'd like some more informations about speculative evolution and molluscs, a quick Google search of the relevant keywords should yield many more pages.

SealBoi
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  • Great answer, just wanted to add a note on this part: It is by no means unreasonable to think that other mollusc groups could A) evolve shells and B) internalize them. As mentioned in the "shell" section of the cephalopod wikipedia article, squids and cuttlefish do have types of internal shells, the cuttlebone for cuttlefish and the gladius for squid. Evolving this into a skeleton with multiple hard parts with joints might be implausible though, maybe better to go the megasquid route and just imagine strong internal muscles for support. – Hypnosifl Aug 01 '19 at 10:45
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It is certainly possible that oceanic mollusk like creatures could grow very large.

On Earth there are stories of sperm whales with circular scars much larger than the suckers of any known giant or colossal squid, and having tentacle pieces in their stomachs much larger than known squids.

Here is a skeptical post showing how big a squid would have to be to have suckers, tentacles or an entire body as long as some stories claim.

http://cameronmccormick.blogspot.com/2007/03/ludicrous-giant-squid-claims.html

Thus it is possible, though not likely, that some really enormous squid actually live in our oceans.

It is certainly possible for smallish octopuses to climb out of water and onto land.

https://news.nationalgeographic.com/2017/10/wales-ceredigion-octopus-land-deaths-spd/1

https://blogs.scientificamerican.com/octopus-chronicles/land-walking-octopus-explained-video/2

http://thekidshouldseethis.com/post/the-abdopus-octopus-walks-across-dry-land-to-hunt-for-crabs3

Clearly it would be just as easy for an octopus like species to evolve into a land dwelling species as it was for fish on Earth to do so.

I think that a larger octopus could walk on land easily if it evolved much stronger muscles in its arms. An elephant's trunk doesn't have any bones, but is strong enough to lift a few hundred ppounds. Thus an alien octopus like being with each arm as strong as an elephant's trunk could weigh as much as the combined weight of all its arms added to the combined extra weight they could lift.

Octopus-like beings over a few hundred or thousand pounds, and perhaps much lighter, would have to evolve bones in their arms and thus have legs of a kind. And then they could eventually evolve to be as large as the largest prehistoric elephants or even the largest prehistoric sauropods that weighed maybe 50 to 200 tons.

But I don't think that giant gastropod like beings could ever move very fast on land, except on the smoothest surfaces which should be very rare. Beings with limbs could probably outrun them on most types of surface - though maybe you can find ways to make them move like the fastest snakes.

M. A. Golding
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